Enter countries at random. If you guess a country that is a member of the club, it will appear in the blue column. If you guess a country that happens to share a border with one of the member countries, it will appear in the green columns. Once you think you know the secret rule, type it in to solve the whole quiz. (Different wordings of the rule are possible, so if it doesn't work first time, just try rephrasing your version of the rule.)